According to a study published by the Imperial College London, it would be possible to reduce the climatic impact of airplanes. In particular, the impact of contrails could be reduced by slightly changing the plane altitude.

Airplanes And Climate Change

The climatic impact of aviation is more and more frequently pointed out. With hundreds of tons of CO2 emitted at each intercontinental flight, planes are one of the most polluting means of transport.

Officially, the IPCC considers aviation is only responsible for 2 to 3% of CO2 emissions. However, the sustained growth of the sector in recent years raises the concerns: aviation could become one of the biggest contributors to the rise in CO2 emissions in the coming years.

In addition, according to some researchers, it would be necessary to remake the calculations – in particular, to take into account the climatic effect of the contrails. Indeed, contrails, i.e., the trails of water vapor released by planes as they cruise at high altitudes, have an effect on climate change via what is called radiative forcing.

It is precisely this radiative forcing effect that researchers at Imperial College London have studied. The research, published in February 2020, suggested it would be possible to greatly reduce the climatic impact of contrails by slightly modifying the flight altitude of airplanes. Let’s understand why.

Condensation Trails, Contrails, And Aircrafts’ Radiative Forcing

The principle behind contrails or vapor rails is that under certain conditions of altitude, pressure, humidity, and temperature, the gases from the combustion of aircraft engines get “condensed”. This forms white bands that can be seen from the ground and which consist essentially of water vapor and combustion residues.

Normally, these streaks dissipate very quickly and have no significant climatic effect. But in some cases, they can transform into more persistent clouds, which remain several hours or even tens of hours suspended in the atmosphere.

The problem is that this artificial cloud cover modifies the energy flows in the atmosphere and accentuates phenomena like the greenhouse effect. Ultimately, it contributes to the disruption of climatic mechanisms and therefore to global warming.

Today, it is estimated that these contrails contribute as much to global warming as all of the CO2 emissions from aircraft. The contribution of the aviation sector to global warming could, therefore, be twice as high compared to estimates from recent years.

Change Flight Altitude To Reduce The Radiative Forcing By Contrails

Researchers at Imperial College sought to study this phenomenon to better understand how and under what conditions contrails were formed. The goal was to then find solutions on how they can have a smaller impact.

According to their work (based on aviation data from Japan), only a small proportion of civil flights, under special conditions, cause the formation of persistent contrails. More specifically, it is around 2.2% of flights that are responsible for 80% of persistent contrails. This means most flights don’t create permanent contrails.

Moreover, in theory, it would be possible to greatly reduce these by slightly changing the cruising altitude of airplanes. In fact, most condensation trails form when hot gases from airplanes are emitted in areas oversaturated with ice (when the relative humidity is higher than saturation). However, despite these zones being generally distributed over large zones horizontally, they are not very significantly developed height-wise. In theory, by flying a little higher or a little lower it seems possible to avoid them.

With the above into consideration, the idea of researchers would be to shift the altitude of flights likely to create contrails (less than 2%). This would be achieved by making planes fly about 600m upwards or downwards (depending on the conditions) in order to also avoid taxing in oversaturated areas.

Altitude And Contrails: A Complex Issue

The idea above needs, however, to be carefully considered to be effective. Traffic changes must be planned to avoid overconsumption of fuel. Technical and safety constraints must also be taken into account – by, for example, examining how altitude changes can take place in areas that are already saturated by air traffic.

By focusing only on the flights most likely to participate in the formation of contrails (less than 2% of flights), the researchers estimate that it is possible to reduce by up to 60% the contrails and the radiative forcing associated. By combining this strategy with the deployment of engines emitting fewer pollutants like DACs (Double Annular Combustors), it becomes possible to go even further. In fact, by reducing the number of residues in expelled gases, the concentration of contrails’ crystallization, which becomes less dense and persistent, is also reduced. All this proved, the combination of these two strategies could significantly reduce the climate impact of aviation.

However, such a strategy would not solve the immense climate problem posed by global air traffic. In fact, air traffic is growing every year. Increasingly low prices, competition from low-cost companies, more and more infrastructures being built… It has never been easier to take the plane.

As a result, CO2 emissions from civil aviation are increasing rapidly. Even by deploying targeted reduction strategies such as those presented by researchers at the Imperial College, such a continued growth will never be compatible with the achievement of the climate objectives set by the IPCC. That’s why besides technical developments, it also becomes crucial to reflect upon the place of air transportation – and in particular to find ways of limiting its use.